The invention relates to a moored offshore or self-propelled floating platform with improved motion characteristics for economic offshore deepwater developments. The floating platform of the present invention is capable of being self propelled in mild environments or moored for use in extreme depths and severe wind and wave conditions.
In the development of offshore energy systems such as deepwater oil and/or gas production, long flowlines, power cables and control umbilicals are frequently required between subsea wells and a host platform. The extended lengths pose energy loss, pressure drop and production difficulties.
Costs of structures for deepwater applications are high and costs are frequently increased due to the foreign locations at which they are fabricated. Other difficulties, associated with deepwater offshore operations, result from floating vessel motions which affect personnel and efficiencies especially when related to liquid dynamics in tanks. The primary motion related problem, associated with offshore petrochemical operations, occurs with large horizontal vessels in which the liquid level oscillates and provides erroneous signals to the liquid level instruments causing shutdown of processing and overall inefficiency for the operation.
Prior art for deepwater has generally resulted in facilities that yield expensive solutions for an offshore oil field development. Such prior art includes tension leg platforms (TLP""s), which may incorporate well drilling capabilities to several mini-TLP designs that perform simplified functions; SPARS which are configured much the same as a spar buoy and have dry wellheads, as opposed to subsea trees; deepwater floating production storage facilities (FPSO""s) and their several variations.
Present tendon technology limits the common TLPs to approximately 5,000-ft water depths, which causes them to be stationed long distances from planned deepwater fields and the subsea wells. Systems that are moored by catenary lines can be placed within the deepwater fields within fairly close proximity to the subsea wells.
The principal elements which can be modified for improving the motion characteristics of a moored floating vessel are the draft, the water plane area and its draft rate of change, location of the center of gravity (CG), the metacentric height about which small amplitude roll and pitching motions occur, the frontal area and shape on which winds, current and waves act, the system response of pipe and cables contacting the seabed acting as mooring elements, and the hydrodynamic parameters of added mass and damping. The latter values are determined by complex solutions of the potential flow equations integrated over the floating vessel""s detailed features and appendages and then simultaneously solved for the potential source strengths. It is only significant to note herein that the addition of features which allow the added mass and/or damping to be xe2x80x98tunedxe2x80x99 for a certain condition requires that several features can be modified in combination, or more preferably independently, to provide the desired properties. The optimization is greatly simplified if the vessel possesses vertical axial symmetry as in the present invention which reduces the 6 degrees of motion freedom to 4, (i.e., roll=pitch=pendular motion, sway=surge=lateral motion, yaw=rotational motion, and heave=vertical motion). It is further simplified if hydrodynamic design features may be de-coupled to linearize the process and ease the ideal solution search.
An object of the present invention is a floating platform which contains features which allow the platform motions to be optimized for size and weight to specified hydrodynamic environments and to include features which reduce offshore oil and gas processing operations and field development costs.
A further object of the present invention is to provide a platform which allows the roll hydrodynamics to be determined and optimized and by other features allows tuning of the frequency response for the vertical heave.
An additional object of the present invention is a more efficient self propelled or severe weather moored deepwater floating platform called an SSP with focus upon providing improved vessel motions in wind and wave conditions while exhibiting features which reduce offshore gas/oil field development and operation costs.
The present invention provides for an offshore floating facility with improved hydrodynamic characteristics and the ability to moor in extended depths thereby providing a satellite platform in deep water resulting in shorter flowlines, cables and umbilicals from the subsea trees to the platform facilities. The design incorporates a retractable center assembly which contains features to enhance the hydrodynamics and allows for the integral use of vertical separators in a quantity and size providing opportunity for individual full time well flow monitoring and extended retention times.
The floating platform of the present invention is capable of being self-propelled in mild environments or moored for use in extreme depths and sever wind and wave conditions. The floating platform may be configured to perform the functions of a well-gathering platform, an offshore utility work platform; a remote power or communication transmission hub or relay platform or a satellite separator platform (SSP). The floating platform is hereafter referred to as an SSP despite its adapted use.
A principal feature of the SSP is a retractable center assembly within the hull, which may be raised or lowered in the field to allow transit in shallow areas. The retractable center assembly provides a means of pitch motion damping, a large volumetric space for the incorporation of optional ballast, storage, vertical pressure or storage vessels, or a centrally located moon pool for deploying diving or remote operated vehicle (ROV) video operations without the need for added support vessels.
Hydrodynamic motion improvements are provided by: the basic hull configuration; extended skirt and radial fins at the hull base; a (lowered at site) center assembly extending the retractable center section with base and mid-mounted hydrodynamic skirts and fins, the mass of the separators below the hull deck of the SSP favorably lowering the center of gravity; and attachment of the steel catenary risers, cables, umbilicals and mooring lines near the center of gravity at the hull base. The noted features improve vessel stability and provide increased added mass and damping which improves the overall response of the system under environmental loading.
Key field production items that are satisfied by the invention are: housing large and efficient vertical high-pressure separators with extended retention times which can minimize multiphase flow with upstream primary separation closer to subsea wells which also improves reservoir recovery ratios, providing vertical separators of such dimension that multiple sensors can be used to optimize the liquid gas interface level; providing more economical full pigging ability with individual control of well flowlines; providing individual well flowline chemical injection without added subsea manifolding; simplification of operations and maintenance requirements; and providing for reduced inspection costs below water by the incorporation of a moon pool on the SSP centerline.
The principal cost reductions evolve from: the ability to perform fabrication in relatively shallow water sites; elimination of the necessity of costly offshore deck installation which is typical of certain deepwater alternatives; allowing for short transportation routes to the offshore field by minimizing draft and allowing use of domestic coastal fabrication facilities; providing duplicate functions for structural appendages to minimize fabricated weight and maximize the available flotation per ton of fabrication; and providing pressure control as close to the field as practical for improved economics of pipeline and flowline steel tonnage and installation cost reduction.
The prior art does not disclose methods for: the use of retractable center sections which by their position, structure, appendages, and contents improve the in-place hydrodynamic characteristics while allowing shallow water access when fully raised. The prior art also does not disclose methods of providing increased extended duration vertical vessels or separators below the floating platform deck that serve to add stability of the floating structure by favorably shifting the center of gravity downward while also increasing the roll and heave-added mass and damping of the floating structure for reduced platform motions in wind and waves. The prior art also does not disclose methods of raising and lowering vertical separators alone or as a unit within a center assembly to allow passage to or from shallow waters. The prior art does not disclose methods of providing a one-atmosphere access zone around the operational components of separators suspended from a floating platform. The prior art does not disclose methods of optimizing a vertical separator""s performance by a nearly continuous array of sensors that allow a variable liquid level. The prior art does not disclose extending a skirt at the base of a hull with a diameter and configuration to ease the offshore attachment of steel catenary risers, umbilicals, and cables.
Because of the features which may be provided for, when utilized in the capacity of a floating deepwater oil and/or gas primary separation platform, the SSP performs bulk separation and yields full time test capabilities of each attached well via the flowlines and well control umbilicals. The hull features of the SSP include objects of the invention such as the vertical columns which provide hull-stiffening while serving as mooring line conduits for above water mooring line tensioning, and tension monitoring; facilities for the installation I addition/maintenance of the long vertical separators offshore; benefits afforded by the hydrodynamic hull damping skirt that doubles as a submerged towing rim and a load ring that distributes the transverse mooring loads and provides a foundation for steel catenary risers; umbilicals and cables; the separator supported damping skirt which doubles as separator spacing restraints; and the separator raising and lowering frame systems allowing shallow water fabrication yard access.
Due to the size of the individual first stage vertical separators and slug catchers afforded by the center assembly space and the available extended residence times which can be accommodated, efficient high-pressure separation can be accomplished for the purposes of minimizing transportation of produced reservoir water over long distances. Vertical, as opposed to horizontal, separators minimize motion effects due to the reduced overall elevation significance of internal separator fluid waves without the need for baffles and utilize the nearly unlimited available space within the water column without wasting deck space.
Other functions provided by the SSP, while in the role of an oil/gas platform, are pigging of the flowlines and outgoing gas/oil lines; metering of multiple wells; chemical injection of the incoming and outgoing lines; manifolding to perform the required functions; quarters facilities for the limited crew required for operations; instrument gas generation and controls; satellite and other information transmissions to the host platform(s); electric power transmission; and umbilical control of the satellite wells.
While Individual prior art includes several of the above features, none have addressed the aspect of extended oil/gas separation residence times in vertical separators and variable elevations control of the liquid gas interface by numerous vertically-spaced sensors, which are a solution to the issue of improved high-pressure separation without excessive treatments. Prior art does not address the issue of minimizing the floating structure draft beyond normal ballasting for fabrication yard and tow conditions to site provided by the SSP. The SSP may therefore be fabricated and/or outfitted at common shipyards. Once lowered, the separators and associated equipment increase the draft beyond normal port access and while doing so, improves the floating vessel stability by lowering the center of gravity and increasing roll and heave-added mass and damping.
The manner of achieving improved hydrodynamic motion improvements within the present invention involves the incorporation of the following features:
1. Use of the vertical reactions of the mooring system and the steel catenary and other risers to achieve an operating draft for the facility which is sufficient for reduced motions with minimal conventional ballast;
2. providing a hull water plane area vs. draft ratio which, when tuned to the heave-added mass and damping, provides sufficient free board in design storms and provides adequate vertical heave damping to resist high-frequency response to normal wind and waves;
3. making use of the difference of the vertical wave particle velocities at the surface and at the base of the center assembly to damp out the storm-induced heave motions;
4. use of the retractable center section weight and its content in the full lowered position to decrease the overall system center of gravity;
5. use of the center section in its lowered position to improve the lateral damping in order to offset lateral motion of the primary hull section and minimize roll and sway motions;
6. use of the trapped hydrodynamic added mass due to the center assembly and its contents as well as enhanced damping and added mass features of spaced segments and the lower center section xe2x80x98skirtsxe2x80x99;
7. use of the skirt extending around the lower circumference of the main hull to decrease the floating roll response by added mass and damping;
8. use of vertical gussets supporting the hull xe2x80x98skirtxe2x80x99 as fins to provide added mass and damping and to reduce yaw rotational motions.
9. providing the opportunity to separately tune roll and heave response frequencies by designing the hull skirt to control roll damping and the hull plus center section hull skirts to control heave added mass and damping, thereby allowing the system to be finely tuned for a drier deck in a design limit sea state.